Fiber-reinforced composite materials in which carbon fibers, aramid fibers, glass fibers or the like are used as reinforcing fibers have been widely utilized for structural materials, such as aircraft and automobiles, and general industry or sports use, such as tennis rackets, golf club shafts and fishing rods, utilizing high specific strength and specific elasticity modulus thereof. As forms of the reinforcing fibers, there are woven fabrics produced by using continuous fibers, UD sheets in which the fibers are aligned unidirectionally, random sheets produced by using cut fibers, nonwoven fabrics and the like.
Generally, in case of the fabrics made of the continuous fibers, the UD sheets and the like, complicated layering steps such as layering at various angles, for example, at 0/+45/−45/90, because of anisotropy of the fibers, and further plane-symmetrical layering for preventing warpage of shaped products, have become one of the factors that increase the cost of the fiber-reinforced composite materials.
Accordingly, a relatively inexpensive fiber-reinforced composite material can be obtained by using a previously isotropic random mat. This random mat can be obtained by a spray-up (dry production method) wherein spraying cut reinforcing fibers alone or spraying the cut fibers together with a thermosetting resin are performed at the same time into a mold, or a paper-manufacturing (wet method) of adding previously cut reinforcing fibers into an aqueous slurry containing a binder, and followed by paper-making process. Use of the dry production method can provide the random mat more inexpensively, because an apparatus is relatively small in size.
As the dry production method, there is commonly used a technique of cutting continuous fibers and concurrently spraying the cut fibers, and a rotary cutter is used in many cases. However, when the distance between blades of the cutter is increased in order to lengthen the fiber length, the cut frequency decreases, and thereby results in discontinuous discharge of the fibers from the cutter. For this reason, the uneven fiber areal weight of the mat locally occurs. In particular, when the mat having a low fiber areal weight of fibers is prepared, the unevenness in thickness becomes significant, which has caused a problem of deteriorated surface appearance.
On the other hand, another factor that increases the cost of the fiber-reinforced composite materials is that the molding time is long. Usually, the fiber-reinforced composite material is obtained by heating and pressurizing a material called a prepreg in which a reinforcing fiber base material is previously impregnated with a thermosetting resin, using an autoclave for 2 hours or more. In recent years, an RTM molding method has been proposed in which a base material of reinforcing fibers not impregnated with a resin is set in a mold, and thereafter, a thermosetting resin is poured thereinto, and the molding time has been substantially reduced. However, even when the RTM molding method is adopted, it takes 10 minutes or more until one part is molded.
For this reason, a composite material using a thermoplastic resin as a matrix, instead of the conventional thermosetting resin, is attracting attention. However, the thermoplastic resin generally has high viscosity compared to the thermosetting resin, so that the time to impregnate the molten resin into the fiber base material becomes long. As a result, there has been a problem that the takt time until molding increases.
As a technique for solving these problems, there is proposed a technique called thermoplastic stamping molding (TP-SMC). This is a molding method in which chopped fibers previously impregnated with a thermoplastic resin are heated to a melting point or more or a flowable temperature or more of the resin and put into a part of a mold, thereafter immediately the mold is closed, and the fibers and the resin are allowed to flow in the mold, thereby obtaining a product shape, followed by cooling to form a shaped product. According to this technique, molding is possible for such a short period of time as about 1 minute by using the fibers previously impregnated with the resin. There are Patent Documents 1 and 2 with respect to methods for producing chopped fiber bundles and molding materials. However, these are methods using molding materials as called SMC or stampable sheets. In such thermoplastic stamping molding, the fibers and the resin are allowed to flow in the mold, so that there have been problems of failing to produce a thin-walled one and fiber orientation is disturbed as the orientation becomes beyond control.
As a means for producing the thin-walled one without allowing the fibers to flow, there is proposed a technique of preparing a thin sheet from reinforcing fibers by a paper-making method, and thereafter, impregnating the sheet with a resin to prepare a prepreg (Patent Document 3). In the paper-making method, the reinforcing fibers are homogeneously dispersed in an aqueous dispersion, so that the reinforcing fibers are in single fiber form.
(Patent Document 1) JP-A-2009-114611
(Patent Document 2) JP-A-2009-114612
(Patent Document 3) JP-A-2010-235779